This invention relates to an expander mechanism for an internal shoe drum brake, particularly for a simplex "expanding-wedge" type brake having a cylinder housing affixed to a brake support, two coaxially mounted pressure pistons slidable in the housing, an expander member joining the pressure pistons, and respective adjusting screws threaded into the pressure pistons, the outer ends of which screws cooperate with flange members of the respective brake shoes, wherein the outer ends of the adjusting screws are each provided with an indentation which has a rounded shape in a cross section perpendicular to the axis of the brake drum, in which recess a pressure piece is inserted having a complementary shape, and which pressure piece, on the side facing away from adjusting screw is slidably connected with the flange member of the brake shoe via a groove in the pressure piece perpendicular to the axis of the brake drum.
In an expander mechanism of this general type, as shown, for example, in East German patent specification No. 2,001,200, the indentations in the exterior ends of the adjusting screws are generally hemicylindrical and the pressure piece is provided with a corresponding generally hemicylindrical configuration. Thus, the support surfaces of the pressure piece and the adjusting screws are cylindrical, with the axis of the cylinder being parallel to the axis of the brake drum. Each pressure piece is provided with a flange member on its side disposed away from the adjusting screw, which flange member engages an approximately radial groove in the flange member of the brake shoe which slidably presses against the base surface of the groove. As the brake lining progressively wears, the brake shoe moves downwardly with respect to the adjusting screw, whereby the plane abutting surface of the pressure piece slides in the groove in the flange member of the brake shoe, while at the same time the pressure piece swings in the hemicylindrical indentation in the associated adjusting screw. In this way, it is intended that for wear of the brake lining, the forces on the adjusting screws and the pressure pistons will act centrally, thus avoiding any torque moment on the adjusting screws and the pressure pistons. A disadvantage of this known expanding mechanism, however, is that in the case of radial expansion of the brake drum, accompanied by changes of alignment parallel to the drum axis, the edges of the pressure pieces are subject to compression and undesirable torques.